Referring to FIG. 1, a conventional window blind disclosed in U.S. Patent Publication no. 2011/0061823 includes an upper rail 1, two roller devices 2 (only one is shown in FIG. 1) disposed or the upper rail 1, two pull cards 3 (only one is shown in FIG. 1) and a blind unit 4. Each of the roller devices 2 includes a bracket 201, and two rollers 202 mounted to the bracket 201. The bracket 201 of each of the roller devices 2 has two end walls 203 spaced apart in a front-rear direction (the front one is omitted in FIG. 1), and each being formed with a plurality open grooves 204, each of which extends downwardly from a top edge of the end wall 203. For each of the roller devices 2, each of the rollers 202 has opposite end portions respectively mounted in an oppositely corresponding pair of the open grooves 204. Each of the pull cords 3 extends around the rollers 202 of a respective one of the roller devices 2, and is connected to the blind unit 4.
The rollers 202 of each of the roller devices 2 are configured to provide a retarding force to the corresponding pull cord 3 when the pull cord 3 is operated to drive movement of the blind unit 4. However, during the operation of each of the pull cords 3, the rollers 202 of the corresponding roller device 2 may move along the corresponding open grooves 204, so that the retarding force provided by the rollers 202 is unsteady. Moreover, since each of the open grooves 204 is formed through the top edge of the corresponding end wall 203, an additional operating module (such as an operating module for operation of a ladder string) can hardly be provided in a space of each of the roller devices 2 located above the rollers 202 thereof.